I have never been able to find a solution to it, unfortunately. But fortunately for me I will be ditching HTC altogether very soon when I get a new phone.
–
sammygApr 16 '13 at 14:18

Now, as for the busy tone problem, if you listen very carefully, preferrably in a very silent room, and you dial a busy phone line you most certainly will be able to hear a very very weak "busy tone". So it's not really true that there is no tone, the busy tone is actually there! But it's completely rubbish and inaudible, so you can't hear it in a busy room or even less when walking down the street so it's completely useless anyway. But it is there. If you are ready to listen carefully for it! ;) Perhaps use a spy kit to be able to hear it.
–
sammygApr 16 '13 at 14:20

I will post back if I find a solution to this. But the only thing that could solve it for sure is rooting the phone and dumping the old ROM for some custom ROM like CyanogenMod.
–
sammygApr 16 '13 at 14:28